The invention relates to an elecro-magnetically operable friction-disk clutch for the impeller of a fan, in particular of motor vehicle internal-combustion engines. The impeller is connected via a first electromagnetic frictiondisk clutch directly to the drive shaft and, at engine speed and with disengaged first friction-disk clutch, is capable of being driven via an eddy-current coupling which can be switched by means of a second friction-disk clutch, to a lower windmilling speed. With disengaged first and second friction-disk clutch, the runner wheel of the eddy-current coupling is capable of being driven via the friction of a ball bearing arranged on the drive shaft.
German patent application DE No. 32 03 143 has disclosed an electromagnetically operable friction-disk clutch for the impeller of a fan, in particular of motor vehicle internal-combustion engines, in which, with disengaged friction-disk clutch, the impeller is carried along by a second clutch device designed as an eddy-current coupling. In this case, the drive shaft is connected to an electrically conductive runner wheel and the freely rotating impeller is connected to a permanent magnet of the eddy-current coupling.
The object and aim of the known friction-disk clutch is to continue moving, at a certain minimum windmilling speed, the impeller of a fan, in particular of a motor vehicle engine, even with de-energized fan drive via an electromagnetically operable friction-disk clutch, in order to ensure the cooling of electronic components in the engine compartment. The impeller of the fan can consequently be driven via an electromagnetically operable friction-disk clutch at the speed of the drive shaft, i.e. at the engine speed, or, with disengaged electromagnetic friction-disk clutch, at a maximum windmilling speed, determined by the eddy-current coupling. In the case of the known arrangement, this windmilling speed is of the order of magnitude of about 1,100 r.p.m. It can be controlled by the number of permanent magnets and by the distance between runner and permanent magnets.
In the case of this known arrangement, sometimes the possibility has been considered to disengage, in special cases the eddy-current coupling completely from the drive shaft, which can take place via a second electromagnetically operable friction-disk clutch or via some other clutch device.
In the case of a further known device, the drive flange for the eddy-current coupling is mounted rotatably, for example by means of a ball bearing, on the engine-driven drive shaft, this drive flange being connected via a second electromagnetic friction-disk clutch to the drive shaft. The direct connection of the drive shaft to the impeller is in this case performed analogously to the abovementioned publication DE No. 32 03 143. At the same time it is necessary here for a further ball bearing to be mounted on the drive flange for the eddy-current coupling, which ball bearing bears the armature ring of the first electromagnetic friction-disk clutch and the impeller connected thereto.
With disengaged first and second friction-disk clutch, the impeller of the known clutch arrangement is driven merely via the friction of the inner ball bearing between drive flange for the eddy-current coupling and drive shaft, an idling speed of about 80 rpm establishing itself. This further development of the device according to DE 32 03 143 consequently has the advantage that the windmilling speed can be reduced still further by separation of the direct connection between the eddy-current coupling and the drive shaft. This gives three adjustable fan speeds for the impeller.
The further development described has, however, the disadvantage that at least two superjacent ball bearings or adjacent ball-bearing pairs are required, which enclose between them the drive flange for the eddy-current coupling as rotatable mounting flange. The air gap at the eddy-current coupling is, for example, of the order of magnitude of 0.8 mm. An air gap change acts directly on the torque transmission of an eddy-current coupling, so that small deviations in the air gap have considerable torque changes as a consequence.
With the use of high-quality, very strong magnets for the permanent magnet of the eddy-current coupling, air gap changes likewise play a great part. As a result of this, the use of two superjacent ball bearings, if appropriate of two superjacent ball-bearing pairs (two adjacent ball bearings in each case) requires a great degree of tolerance keeping, in particular also with regard to the drive flange in between for the eddy-current coupling. Another disadvantage of the known further development is that superjacent ball-bearing pairs also have a great space requirement which greatly increases the overall electromagnetically operable friction-disk clutch in its dimensions. Furthermore, in the case of a plurality of individual ball bearings, the possibility arises of stresses occurring due to imprecise fabrication, which leads to premature wear and an unsatisfactory clutch action.